The Inner Sea of the Maldives reveals the history of the Indian monsoon

21.12.2015

Scientists have just returned from an expedition to the Inner Sea of the Maldives where they recovered 3097 m of cores that contain the history of the monsoon and the ocean current system. The international team of scientists retrieved rocks from buried reefs that reveal the growth and demise of ancient coral reefs and sediments from ocean currents that reveal the onset and fluctuations of the Indian monsoon.

Ocean drilling research vessel JOIDES Resolution
CEN scientists from the Institute for Geology took part at the IODP expedition 359: From left, Jesus Reolid, Christian Betzler and Thomas Lüdmann in front of the research vessel JOIDES Resolution in Colombo, Sri Lanka

The monsoon is one of the most dramatic recurring weather phenomenon on Earth and affects over a billion people every year. Yet, little is known when the monsoon started and how it changed over time, or if it could change again in the future. The monsoon brings rainfall to the continents for agriculture and on the ocean it is known for its winds that change direction with the winter and summer monsoon. For many years scientist have tried to reconstruct the monsoon history from the rain-induced weathering and discharge into the ocean. In the expedition onboard the research vessel JOIDES Resolution a novel approach was taken to extract the history from wind related features.

Today, the winds of the monsoon drive the ocean currents across the Maldives. These currents, like rivers in the ocean carry sediment. In the Inner Sea of the Maldives the currents slow down and release the sediment to build large drift deposits. The sediments in these drifts hold the record of climate change and monsoon activity for the last 12 million years. The sediments, however, also bury ancient reef buildups that flourished in the Inner Sea before the monsoon started. These reefs carry the history of sea level changes before the onset of the monsoon.

 An international team of 30 scientists from 15 countries has just returned from an eight-week scientific expedition onboard the JOIDES Resolution to collect core samples of these sediments and the buried reefs. Although the Maldives have long been recognized as prime sites to study coral growth, International Ocean Discovery Program (IODP) Expedition 359: Maldives Monsoon and Sea Level is the first to retrieve sediment core samples from deep below the seafloor here.

“We have unraveled the physical evidence of the monsoon and now know the exact timing when the modern monsoon pattern began, and have shown what consequences the onset of the monsoon had on the coral reefs of the Maldives,” says Christian Beztler, Co-Chief Scientist for Expedition 359, from the CEN at the University of Hamburg in Germany. “The scientific results of this expedition will give answers to many fundamental questions of the monsoon and the climate in general.”

A period of global cooling preceded the onset of the monsoon

The team found evidence for a period of global cooling that preceded the onset of the monsoon. This led to an expansion of the West Antartic Ice sheet that was responsible for global sea level fall that exposed many reefs in the Maldives. During this global cooling period the monsoon started and currents invaded the Inner Sea of the Maldives. The sediment that was carried by the current buried several reefs. The currents also cause local upwelling that were again detrimental for the reefs. At three of the eight drill sites, these drowned reefs were found covered by current deposits.

"Not only the upwelling, which injected nutrients harmed the reefs, but also the strength of the currents sweeping over the reef flats and along the flanks of the reefs were detrimental for the corals and the reef edifices" states Gregor Eberli, Co-Chief Scientist of Expedition 359, from the University of Miami in the USA.

 The scientific party of the cruise was very surprised to discover that the current pattern changed many times indicating re-organizations and changes in the monsoonal changes. Such changes are most likely caused by the variations of the earth orbital pathes.

The scientists are looking forward to work out the details of the monsoonal evolution that is essential for so many people in Asia and India. This will be done analyzing the many samples recovered during this expedition.

The International Ocean Discovery Program (IODP) is an international research collaboration that coordinates seagoing expeditions to study the history of the Earth recorded in sediments and rocks beneath the ocean floor. The JOIDES Resolution Science Operator (JRSO) operates the scientific drillship JOIDES Resolution on behalf of the National Science Foundation.

More information

IODP Expedition 359: Maldives Monsoon and Sea Level